The scale on a map is 9cm : 3km. If two cities are 81cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 9cm on the map corresponds to an actual distance of 3km. A distance of 81cm on the map is the same as 9 $\cdot$ 9cm. The actual distance between the two cities is 9 $\cdot$ 3km, or 27km.